rtunately, with this apparatus the determinations of water
are very exact, and since the amount of water vaporized inside the
chamber is known it is possible to compute the heat required to vaporize
this water by knowing the heat of vaporization of water.

Since the earlier reports describing the first form of calorimeters were
written, there has appeared a research by one of our former associates,
Dr. A. W. Smith[11] who, recognizing the importance of knowing exactly
the heat of vaporization of water at 20 deg., has made this a special
object of investigation. When connected with our laboratory a number of
experiments were made by Doctors Smith and Benedict in an attempt to
determine the heat of vaporization of water directly in a large
calorimeter; but for lack of time and pressure of other experimental
work it was impossible to complete the investigation. Subsequently Dr.
Smith has carried out the experiments with the accuracy of exact
physical measurements and has given us a very valuable series of
observations.

Using the method of expressing the heat of vaporization in electrical
units, Smith concludes that the heat of vaporization of water between
14 deg. and 40 deg. is given by the formula

L (in joules) = 2502.5 - 2.43T

and states that the "probable error" of values computed from this
formula is 0.5 joule. The results are expressed in international joules,
that is, in terms of the international ohm and 1.43400 for the E.M.F. of
the Clark cell at 15 deg. C., and assuming that the mean calorie is
equivalent to 4.1877 international joules,[12] the formula reads

L (in mean calories) = 597.44 - 0.580T

With this formula Smith calculates that at 15 deg. the heat of vaporization
of water is equal to 588.73 calories; at 20 deg., 585.84 calories; at 25
deg., 582.93 calories; at 30 deg., 580.04 calories;[13] and at 35 deg.,
577.12 calories. In all of the calculations in the researches herewith we
have used the value found by Smith as 586 calories at 20 deg. Inasmuch as
all of our records are in kilo-calories, we multiply the weight of water
by the factor 0.586 to obtain the heat of vaporization.

THE BED CALORIMETER.

The chair calorimeter was designed for experiments to last not more than
6 to 8 hours, as a person can not remain comfortably seated in a chair
much longer than this time. For longer experiments (experiments during
the night and particularly for bed-ridden patients) a type of
calorime